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The water injection is one of the recognized technologies capable of helping the future engines to work at full load conditions with stoichiometric mixture. In the present work, a methodology for the CFD simulation of reacting flow conditions using AVL Fire code v. 2020 is applied for the assessment of the water injection effect on modern GDI engines. Both Port Water Injection and Direct Water Injection have been tested for the same baseline engine configuration under reacting flow conditions. The ECFM-3Z model adopted for combustion and knock simulations have been performed by adopting correlations for laminar flame speed, flame thickness and ignition delay times prediction, to consider the modified chemical behavior of the mixture due to the added water vapor. This improved methodological approach allows considering both the fluid-dynamics aspects (in terms of turbulence level close to ignition time and average in the combustion chamber), the mixture quality and the chemical properties of the mixture (first of all the laminar flame speed) related to the water injection. The main result for the design process is the need of finding the best tradeoff between the cooling of the unburnt mixture (which reduces the knock risk), the need to preserve both the turbulence and the laminar flame speed (which is already penalized by the cooling of the charge).

The water injection is one of the technologies assessed in the development of new internal combustion engines fulfilling new emission regulation and policy on Auxiliary Emission Strategy assessment. Besides all the positive aspects about the reduction of mixture temperature at top dead centre and exhaust gases temperature at turbine inlet, it is well known that the water vapour acts as a mixture diluter, thus diminishing the reactants burning rate. A common methodology employed for the RANS CFD simulation of the reciprocating internal combustion engines turbulent combustion relies on the flamelet approach, which requires the knowledge of the laminar flame speed and thickness. Typically, these properties are calculated by mean of correlation laws, but they do not keep into account the presence of water mass fraction. A more precise methodology for the definition of both the laminar flame speed and thickness is thus required. The interrogation of a previously computed look-up table of such properties during run time seems to be a suitable and more accurate method than using correlations. In order to generate a database with all the possible combinations of chemical and physical properties that can be reached during the simulation of internal combustion engines, including the presence of a given mass fraction of water vapour and exhaust gases, a very high number of detailed chemical kinetics simulations needs to be performed. The present work aims to introduce a new methodology for the fast generation of laminar flame characteristics look-up tables that account also for the presence of water vapour in the reacting mixture. By using this new approach, engine designers will have the possibility to generate look-up tables of laminar flame characteristics for different fuels with the same computational cost that is currently required to generate a single table.

Water injection is becoming a technology of increasing interest for SI engines development to comply with current and prospective regulations. To perform a rapid optimization of the main parameters involved by the water injection process, it is necessary to have reliable CFD methodologies capable of capturing the most important phenomena. In the present work, a methodology for the CFD simulation of combustion cycles of SI GDI turbocharged engines under water injection operation is proposed. The ECFM-3Z model adopted for combustion and knock simulations takes advantages by the adoption of correlations for the laminar flame speed, flame thickness and ignition delay times prediction obtained by a detailed chemistry calculation. The latter uses machine learning algorithms to reduce the time to generate the full database while still maintaining an even distribution along the variables of interest. The results demonstrate the applicability of the proposed methodology, capable of capturing not only the thermodynamic effects of water injection but also the chemical kinetics aspects related to the mixture water dilution whose prediction is mandatory for addressing the engine design according to different goals: complying with new emission directives and limits, turbine inlet temperature constraints, minimization of the BSFC and possibly engine power increase.

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